Method For The Printing Of Homogeneous Electronic Material With A Multi-Ejector Print Head

ABSTRACT

Printing systems are disclosed that produce homogenous, smooth edged printed patterns (such as integrated circuit (IC) patterns) by separating pattern layouts into discrete design layers having only parallel layout features. By printing each design layer in a printing direction aligned with the parallel layout features, the individual print solution droplets deposited onto the substrate do not dry before adjacent droplets are deposited. Therefore, printed patterns having accurate geometries and consistent electrical properties can be printed.

RELATED APPLICATIONS

Further, this application is a continuation of U.S. patent applicationSer. No. 11/019,038, entitled “Method For The Printing Of HomogeneousElectronic Material With A Multi-Ejector Print Head” filed Dec. 20, 2004which is a continuation of U.S. patent application Ser. No. 10/224,701,entitled “Method For The Printing Of Homogeneous Electronic MaterialWith A Multi-Ejector Print Head”, filed Aug. 20, 2002 now U.S. Pat. No.6,890,050.

This invention was made with Government support under 70NANBOH3033awarded by NIST/ATP. The Government has certain rights in thisinvention.

FIELD OF THE INVENTION

This invention relates generally to electronic materials processing, andmore particularly to a method and system for creating and operating ahigh-resolution printing system.

BACKGROUND OF THE INVENTION

Integrated circuit (IC) printing is an emerging technology that attemptsto reduce the costs associated with IC production by replacing expensivelithographic processes with simple printing operations. By printing anIC pattern directly on a substrate rather than using the delicate andtime-consuming lithography processes used in conventional ICmanufacturing, an IC printing system can significantly reduce ICproduction costs. The printed IC pattern can either comprise actual ICfeatures (i.e., elements that will be incorporated into the final IC,such as the gates and source and drain regions of thin film transistors,signal lines, opto-electronic device components, etc.) or it can be amask for subsequent semiconductor processing (e.g., etch, implant,etc.).

Typically, IC printing involves depositing a print solution (generallyan organic material) by raster bitmap along a single axis (the “printtravel axis”) across a solid substrate. Print heads, and in particular,the arrangements of the ejectors incorporated in those print heads, areoptimized for printing along this print travel axis. Printing of an ICpattern takes place in a raster fashion, with the print head making“printing passes” across the substrate as the ejector(s) in the printhead dispense individual droplets of print solution onto the substrate.At the end of each printing pass, the print head makes a perpendicularshift relative to the print travel axis before beginning a new printingpass. The print head continues making printing passes across thesubstrate in this manner until the IC pattern has been fully printed.

Once dispensed from the ejector(s) of the print head, print solutiondroplets attach themselves to the substrate through a wetting action andproceed to solidify in place. The size and profile of the depositedmaterial is guided by competing processes of solidification and wetting.In the case of printing phase-change materials for etch mask production,solidification occurs when the printed drop loses its thermal energy tothe substrate and reverts to a solid form. In another case, colloidalsuspensions such as organic polymers and suspensions of electronicmaterial in a solvent or carrier are printed and wet to the substrateleaving a printed feature. The thermal conditions and materialproperties of the print solution and substrate, along with the ambientatmospheric conditions, determine the specific rate at which thedeposited print solution transforms from a liquid to a solid.

If a first droplet and a second adjacent droplet are applied onto thesubstrate within a time prior to the phase transformation of the firstdroplet, the second droplet will wet and coalesce to the first dropletin its liquid or semi-liquid state to form a continuous printed feature.Print solutions having high surface tension will also beneficiallyprevent the next overlapping droplet from spreading on the substratesurface, thus minimizing the lateral spreading of the droplets. FIG. 1 ashows a photograph of a printed feature 10 a that was printed in asingle printing pass in the X axis direction. Because adjacent dropletsdeposited during the single printing pass did not have time to drybetween ejection events, feature 100 a exhibits the desired homogeneityand smooth side wall profiles that result from optimal dropletcoalescence. In contrast, FIG. 1 b shows a photograph of a printedfeature 100 b formed by raster printing in the Y axis direction. Feature100 b therefore represents a “multi-pass” feature; i.e., a printedfeature formed by multiple passes of the print head. In a multi-passfeature, the droplets deposited during sequential passes of the printhead are typically dry before any adjacent droplets from the nextprinting pass are deposited. Consequently, the drops of print solutionthat make up the multi-pass feature are not able to coalesce andtherefore create “scalloped” feature borders. This edge scalloping canbe seen in FIG. 1 b, as the individual print solution droplets 110 bused to form feature 100 b are all clearly visible.

Typically, an IC pattern includes both multi-pass features and featuresthat are aligned with the print direction. FIG. 1 c shows a photographof an IC pattern 100 c printed using a conventional IC printingprocess—in this case a raster printing operation in the Y axisdirection. IC pattern 100 c is made up of an array of transistorelements 120 interconnected by multiple address lines 160 and word lines170. Word lines 170, which run parallel to the Y axis and were thereforealigned with the print direction, exhibit the desirable homogeneity andsmooth sidewalls described with respect to FIG. 1 a. However, addresslines 160, which are printed by multiple printing passes in the Y axisdirection, all exhibit the undesirable edge scalloping andnon-coalescence described with respect to FIG. 1 b.

The edge scalloping shown in FIGS. 1 b and 1 c is related to a varietyof problematic issues. For example, if the IC pattern is a mask, theirregular edges of feature 100 b can result in unreliable print qualityand patterning defects leading to inconsistent device performance.Perhaps more significantly, edge scalloping in an actual IC featureindicates a potentially serious underlying defect. The electronicbehavior of an IC feature is affected by its molecular structure. Inparticular, the molecules of organic printing fluids are typically longchains that need to self-assemble in a particular order. However, if adroplet of such printing solution solidifies before an adjacent dropletis deposited, those chains are not allowed to properly assemble, leadingto a significant reduction in the electrical continuity between the twodroplets. This in turn can severely diminish the performance of thedevice that incorporates the scalloped printed feature.

What is needed is a system and method for accurately printing ICpatterns that allows the printed features to be optimized for edgeprofile and electrical continuity.

SUMMARY OF THE INVENTION

The present invention is directed towards printing systems that can beused to produce homogenous printed patterns on a substrate. Byseparating a pattern layout into discrete design layers having onlyparallel layout features, a printed pattern can be formed by a series ofprinting operations, wherein the print direction of each printingoperation is aligned with the parallel layout features of the designlayer being printed. In this manner, the printing of multi-pass featurescan be avoided, and homogenous, smooth-edged printed patterns can beprinted.

According to an embodiment of the invention, the print head in aprinting system can have ejectors spaced according to the design rulesof the pattern being printed, thereby enhancing printing throughput byminimizing the amount of perpendicular shifting required betweenprinting passes. According to another embodiment of the invention, thedesign rules of a pattern layout can be formulated according to theknown ejector spacing in a print head to be used in printing the IClayout.

According to another embodiment of the invention, a print head can bedesigned to print in multiple directions by arranging the ejectors in apattern conducive to multi-directional printing (i.e., arranging theejectors in a pattern other than a straight line perpendicular to theprint direction). Various ejector arrangements can be incorporated intoa print head to enhance the printing capability of a printing system.

A printing system can be calibrated to provide optimal accuracy bycomparing the actual location of printed spots with their design(expected) locations and adjusting the printing system accordingly. Forexample, according to an embodiment of the invention, rotationalmisalignment of the print head to the stage can be detected andcorrected by printing test spots from two different ejectors at the samedesign location, and then rotating the print head to compensate for anyoffset between the actual printed spots. According to another embodimentof the invention, test spots from all the ejectors could be dispensed atonce and compared against their expected locations. A graph of thelinear offsets can then be used to calculate an angular offset for theprint head.

According to another embodiment of the invention, test spots from allthe ejectors in a print head can be used to compensate for thermallyinduced changes in ejector position. By graphing the horizontal andvertical offsets from expected ejector positions, a thermal correctionfactor can be derived for calculating the actual ejector positions fromthe design ejector positions.

According to another embodiment of the invention, printing operationsinvolving different design layers can be aligned by initially printingalignment marks on the substrate. By calibrating the IC printing systemagainst those alignment marks prior to printing new design layers,alignment between the IC features printed from the different designlayers can be maintained.

According to another embodiment of the invention, the ejectors in aprint head can be selection-filtered so that those ejectors that cannotprovide a minimum level of printing accuracy are not used. According toan embodiment of the invention, a rasterized image can be efficientlyprinted with the print head in this “sparse” ejector condition byaligning the first active ejector with the first raster column of therasterized image. This in turn aligns any other active ejectors withother raster columns, and the data from the raster columns aligned withactive ejectors are printed. The first active ejector is then shifted tothe next raster column that contains valid data, thereby aligning theother active ejectors with new raster columns. The new data is thenprinted, and the process is continued until no valid data remains in therasterized image.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects and advantages of the presentinvention will become better understood with regard to the followingdescription, appended claims, and accompanying drawings, where:

FIG. 1 a is a photograph of a printed feature exhibiting smooth edgesand homogeneity;

FIG. 1 b is a photograph of a printed feature exhibiting scallopededges;

FIG. 1 c is a photograph of an IC pattern including features havingsmooth edges and features having scalloped edges;

FIG. 2 is a perspective view showing a printing system according to anembodiment of the invention;

FIG. 3 a is a sample IC layout;

FIGS. 3 b and 3 c are design layers taken from the sample IC layout ofFIG. 3 a, in which all the layout elements are parallel to one another,according to another embodiment of the invention;

FIG. 3 d is a photograph of an IC pattern formed using the design layersshown in FIGS. 3 b and 3 c, according to another embodiment of theinvention;

FIGS. 4 a, 4 b, and 4 c are facing views of print heads in accordancewith other embodiments of the invention;

FIGS. 5 a and 5 b are diagrams of a print head to print travel axisalignment process according to another embodiment of the invention;

FIG. 5 c is a flow diagram of the print head to print travel axisalignment process shown in FIGS. 5 a and 5 b, according to anotherembodiment of the invention;

FIGS. 6 a and 6 b are diagrams of a print head to print travel axisalignment process according to another embodiment of the invention;

FIG. 6 c is a flow diagram of the print head to print travel axisalignment process shown in FIGS. 6 a and 6 b, according to anotherembodiment of the invention;

FIG. 7 is a graph of rotational misalignment for use in a stage to printhead alignment process according to another embodiment of the invention;

FIGS. 8 a and 8 b are graphs of horizontal and vertical misalignmentthat can be used to determine thermal effects on ejector positionaccording to another embodiment of the invention;

FIG. 8 c is a flow diagram of an ejector mapping process according toanother embodiment of the invention;

FIGS. 9 a and 9 b are diagrams of layer alignment processes according toother embodiments of the invention;

FIGS. 9 c and 9 d are flow diagrams of the layer alignment processesshown in FIGS. 9 a and 9 b, respectively, according to other embodimentsof the invention;

FIGS. 10 a and 10 b are diagrams of a procedure for printing with sparseejectors according to another embodiment of the invention;

FIG. 10 c is a flow diagram of the procedure for printing with sparseejectors shown in FIGS. 10 a and 10 b, according to another embodimentof the invention; and

FIG. 11 is a flow diagram of a calibration and printing process for aprinting system according to another embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 2 is a perspective view of a printing system 200 in accordance withan embodiment of the invention. Note that while the embodiments of theinvention are described with respect to IC printing for explanatorypurposes, the invention can be applied to any situation in whichhomogenous, smooth-walled features in a printed pattern are required.Printing system 200 includes a stage 210 for supporting (and optionallytranslating) a substrate 220, a print assembly 250 mounted to a printingsupport structure 280, and a computer/workstation 290 that serves asboth a system controller and data processor. Stage 210 includes arotational platform 212 that allows the orientation of substrate 220 tobe adjusted. Optional alignment features 211 on rotational platform 212can be included to provide gross positioning and capture of substrate220. Print assembly 250 includes a print head 230 (on a rotationalfixture) and a camera 270 (having high magnification capabilities)mounted in a rigid mount 260. Print head 230 includes one or moreejectors 240 mounted in an ejector base 231. Ejectors 240 are configuredto dispense droplets of a printing fluid on substrate 220. Depending onthe type and intended use of the printed pattern being formed, theprinting fluid can comprise a variety of materials, includingphase-change materials such as wax or photoresist (to form semiconductorprocess masks), and colloidal suspensions such as solution-processableelectronic (i.e., conducting, semiconducting, or dielectric) materials,and organic or inorganic materials (e.g., to form IC features).Substrate 220 can comprise any material on which patterning can beperformed, such as a wafer, a glass plate, or even flexible materialssuch as fabric or plastics. As will be discussed subsequently, ejectors240 can be in various arrangements and orientations, according tovarious embodiments of the invention.

Computer/workstation 290 is configured to receive IC layout data from adata source 291, and then provide appropriate control signals toprinting support structure 280 and/or stage 210. Data source 291 cancomprise any source of IC layout data, including a networked computer, alayout database connected via a local area network (LAN) or wide areanetwork (WAN), or even a CD-ROM or other removable storage media. Thecontrol signals provided by computer/workstation 290 control the motionand printing action of print head 230 as it is translated relative tothe substrate 220. Note that the printing action can be provided byprinting support structure 280, by stage 210, or by both in combination.Note further that the printing action does not have to involve actualmovement of the print head itself, as print head 230 could be heldstationary while stage 210 translates substrate 220.Computer/workstation 290 is also coupled to receive and process imagingdata from camera 270. As will be described subsequently, camera 270 canprovide both manual and automated calibration capabilities for printingsystem 200.

To obtain the desired IC pattern results from printing system 200, theIC layout data must be appropriately processed, print head 230 must beproperly configured, and print head 230 must be accurately aligned andcalibrated with respect to stage 210.

IC Layout Data Processing

As noted previously, conventional IC printing systems typically producemulti-pass features that exhibit problematic edge scalloping (and theassociated undesirable electrical properties). According to anembodiment of the invention, the need to print multi-pass features canbe minimized or eliminated by separating the source IC layout intodesign layers that only include parallel layout features. An IC layoutis typically well suited to this sort of division, since mostconventional circuitry is formed using orthogonal rectangular features.Current flow typically follows those orthogonally oriented features,making electrical continuity in those directions particularly importantwith regard to device performance.

Parsing the IC layout into design layers having parallel layout featureseliminates multi-pass features by allowing the print direction of the ICprinting system to always be aligned with the layout features beingprinted. As described previously, printing in a direction parallel to anIC layout feature allows the deposited print solution droplets to fullycoalesce, such that the resulting IC pattern feature is smooth-edged andstructurally homogenous. Note that computer/workstation 290 couldperform this IC layout separation operation, or IC layout data source291 could provide “pre-separated” layout data to computer/workstation290.

FIG. 3 a shows an example IC layout 300 a that includes an array oftransistor elements 310 interconnected by multiple address line elements320 and word line elements 330. Each of transistor elements 310 includesa gate element 311, a drain element 312, and a source element 313.Because IC layout 300 a includes layout features running parallel toboth the X axis and the Y axis, printing IC layout 300 a usingconventional IC printing systems would result in the printing ofmulti-pass features, regardless of whether the print direction wasparallel to the X axis or parallel to the Y axis. The invention avoidsthis problem by separating IC layout 300 a into multiple design layers,such as design layers 300 b and 300 c, shown in FIGS. 3 b and 3 c,respectively. Design layer 300 b in FIG. 3 b includes all the word lineelements 330 of IC layout 300 a, which run parallel to the Y axis, whiledesign layer 300 c in FIG. 3 c includes all the address line elements320 of IC layout 300 a, which run parallel to the X axis. Note that theterms “X axis” and “Y axis” as used herein merely describe twoorthogonal axes, and do not specify any absolute frame of reference.

Once design layers 300 b and 300 c have been extracted from IC layout300 a, the word and address lines of the final IC pattern can be printedin two separate printing operations. In a first printing operation, theprint direction can be set parallel to the Y axis to print design layer300 b onto a substrate. Because design layer 300 b only includes layoutelements that run parallel to the Y axis, setting the print directionparallel to the Y axis will result in smooth-edged, structurallyhomogenous printed features providing good electrical continuity. In asecond printing operation, the print direction can be set parallel tothe X axis to print design layer 300 c onto the substrate. Becausedesign layer 300 c only includes layout elements that run parallel tothe X axis, setting the print direction parallel to the X axis willagain result in smooth-edged, structurally homogenous printed featuresproviding good electrical continuity. The order of printing could bereversed, so long as the print direction associated with each designlayer is maintained. In a similar manner, drain elements 312 and sourceelements 313 could be printed by a third printing operation having aprint direction parallel to the X axis, while gate elements 311 could beprinted by a fourth printing operation having a print direction parallelto the Y axis.

The “print direction,” as used herein, refers to a specific axisrelative to the substrate along which printing occurs. Therefore, twodifferent print directions are associated with the first and secondprinting operations described above. This is true even if the printtravel axis (i.e., the axis of movement of the print head relative tothe printing system), or print travel axes, remain the same for both thefirst and second printing operations. As long as the rotationalorientation of the substrate relative to the print head is differentduring the first and second printing operations, the two printingoperations will have different print directions. Note further that sincethe “print direction” refers to an axis, both positive and negativemotion along that axis are considered to be considered to have the samethe print direction. Also note that the print direction can comprise acurvilinear geometry (i.e., a non-straight-line axis) specifying aplanar or non-planar path. Furthermore, while an IC layout may not bedivisible into design layers having only parallel layer features, solong as substantially all (generally 90% or more) of the layout featuresin a particular layer are parallel to one another, a significant benefitcan be achieved.

FIG. 3 d shows a photograph of an IC pattern 300 d that could be printedfrom design layers 300 b and 300 c in FIGS. 3 b and 3 c, respectively.Note that word lines 360 of IC pattern 300 d, which were formed by aprinting operation having a print direction parallel to the Y axis, allexhibit the desired smooth edges and homogeneity previously describedwith respect to printed feature 100 a shown in FIG. 1 a. Similarly, notethat address lines 370, which were formed by a printing operation havinga print direction parallel to the X axis, are likewise smooth-edged andhomogenous. In this manner, division of IC layouts into appropriatedesign layers can enable improved printing of IC patterns. Contrast thesmooth-edges of IC pattern 300 d with the scalloped edges of IC pattern100 c shown in FIG. 1 c, which was formed using a conventional printingmethod.

Print Head Configuration

Beyond dividing the IC layout into design layers having parallel layoutfeatures, the IC printing process can be further optimized throughproper configuration of the print head. According to an embodiment ofthe invention, printing throughput can be enhanced by spacing theejectors in the print head according to the design rules of the ICpattern being printed. For example, FIG. 4 a shows a facing view (i.e.,looking directly into the ejectors) of a print head 430 a. Print head430 a includes ejectors 440(1)-440(5) mounted in an ejector base 431 a.Note that while five ejectors are shown for explanatory purposes, printhead 430 a could include any number of ejectors. Print head 430 a isdesigned to print in the X axis direction, as ejectors 440(1)-440(5) arearranged in a vertical line. By sizing a spacing Ha between ejectors440(1)-440(5) to be an integer multiple of a selected one of the designrules of a particular IC layout, print head 430 a can be optimized forefficient printing of that IC layout.

For instance, IC layout 300 a shown in FIG. 3 a could include a designrule specifying the minimum spacing between address lines to be 338 um,in which case the spacing d1 between address line elements 320 could be338 um. By setting spacing Ha of print head 430 a to equal 338 um, onlya single pass of print head 430 a across the substrate would be requiredto print design layer 300 c shown in FIG. 3 c. During the printing pass,each of ejectors 440(1)-440(4) would print one of address line elements320. (Note that ejector 440(5) could be disabled during the printingpass, since it would never correspond to any of the layout features indesign layer 300 c.) Therefore, by incorporating an appropriate ejectorspacing into print head 430 a, the total number of passes required toprint a given IC layout can be minimized. According to anotherembodiment of the invention, a similar printing efficiency could beobtained by specifying a design rule(s) for an IC layout that matchesthe (known) spacing of ejectors in the print head to be used in theprinting of the IC layout.

Note that in order to print an orthogonal pair of design layers (such asdesign layers 300 b and 300 c shown in FIGS. 3 b and 3 c), a print headsuch as print head 430 a would require a 90 degree reorientationrelative to the substrate between printing operations (either byrotating the print head or rotating the substrate). The print heads usedin conventional IC printing systems would also require this sort ofrotational reorientation between printing operations since conventionalIC printing systems typically use the same type of print heads that areused in inkjet printers (such as those manufactures by Epson, HewlettPackard, Tektronics, Xerox, etc.), which are optimized for printingalong a single print travel axis (since the print head in an inkjetprinter only moves along a single axis during drop ejection). This typeof inter-layer rotation can introduce undesirable operational complexityand an increased potential for misalignment into the IC printingprocess. According to another embodiment of the invention, the need forprint head/substrate rotation between printing operations can beeliminated by configuring the print head to be a bi-axial print head.

FIG. 4 b shows a bi-axial print head in accordance with anotherembodiment of the invention. Print head 430 b includes multiple ejectors440 arranged in a diagonal line across an ejector base 431 b. Note thatwhile six ejectors are shown for explanatory purposes, print head 430 bcould include any number of ejectors. The diagonal ejector arrangementof print head 430 b allows multi-line printing to be performed in boththe X axis and Y axis directions without print head or substraterotation. Note that the throughput capability of print head 431 b can befurther optimized by setting the horizontal spacing Hb and the verticalspacing Wb between ejectors 440 according to the design rules of the IClayout being printed, as described previously with respect to print head430 a shown in FIG. 4 a.

Print head 430 b can be further optimized by repeating the diagonal lineof ejectors in orthogonal directions, but offsetting each repeated lineby a specified increment to cut down on the number of passes required tofill out an IC pattern. For example, FIG. 4 c shows a print head 430 cthat includes five diagonal lines (D1-D5, shown as dotted lines forreference) of four ejectors 440 each. Note that both the quantity ofdiagonal lines and the number of ejectors per line have been arbitrarilyselected for explanatory purposes, and print head 430 c could includeany number of diagonal lines and ejectors. For printing in the X-axisdirection, diagonal lines D1-D3 are offset from one another in thevertical direction by a distance Hc. Offset Hc is selected such thateach ejector 440 in diagonal lines D1-D3 has a unique vertical positionand can therefore produce distinct (i.e., non-overlapping) horizontallines when the print direction of print head 430 c is parallel to the Xaxis. Similarly, for printing in the Y-axis direction, diagonal linesD3-D5 are offset from one another in the horizontal direction by adistance Wc. Offset Wc is selected such that each ejector 440 indiagonal lines D3-D5 has a unique horizontal position and can producedistinct vertical lines when the print direction of print head 430 c isparallel to the Y axis. In this manner, the printing resolution (i.e.,the printable spacing between adjacent lines) of print head 430 c can besignificantly enhanced.

System Calibration

To ensure accurate IC pattern formation, proper calibration of the ICprinting system is important. Even state of the art inkjet printingheads and ejectors, such as those made by Spectra, Inc., may onlyprovide an ejector-to-ejector spot placement accuracy of 50 um, whereasa more desirable accuracy for the formation of electronic patterns is inthe range of 1 um. Therefore, calibration techniques that can enhancethe “off the shelf” performance of print heads are desirable. As notedpreviously, system calibration can be readily accomplished with a videocamera microscope (such as camera 270 shown in FIG. 2) having an opticalaxis position that is fixed relative to the ejector positions of theprint head. FIG. 11 shows a flow chart of a calibration and printingprocess in accordance with an embodiment of the invention. In step 1110,the position of the ejectors relative to the camera is determined. Thenin step 1120, the print head is aligned with the print travel axis (oraxes) of the printing system. Note that adjustments made in step 1120can require repetition of the camera-head positioning of step 1110. Instep 1130, the “in use” positions of the ejectors (taking into accountthermal expansion of the print head) are determined. In step 1140, theejectors are selection-filtered so that only those ejectors providing adesired degree of printing accuracy are used in the printing process. Ifthe ejector(s) used in the camera-head positioning of step 1110 are notselected to be used in step 1140, another camera-head positioningoperation is performed in step 1150 using an ejector(s) selected in step1140. An optional verification step 1160 can then be performed usingthose ejectors selected in step 1140 to verify that the proper printingaccuracy is provided by the calibrated printing system. During an actualpattern printing operation, a first design layer is printed along with aset of alignment marks in step 1170. All subsequent design layers arethen aligned and printed in step 1180. Note that various otherembodiments of the invention can include any combination of the stepsshown in FIG. 11. The individual steps in the flow chart of FIG. 11 arediscussed in greater detail below.

Camera-Head Positioning (Step 1110)

The first step in any calibration operation using such a camera requiresthat the position of the camera relative to the print head be accuratelydetermined. This “camera-to-print-head position” determination can bereadily made since both the camera and print head are held in fixedpositions relative to each other. For example, FIG. 5 a (to be discussedin greater detail subsequently) shows a camera 570 and print head 531mounted in a print assembly 550. One way to determine the position ofcamera 570 relative to a selected ejector (e.g., ejector 540(0)) is toprint a spot (e.g., 525(0)) from the selected ejector and measure thehorizontal offset Ch and vertical offset Cv required to position areference mark 571 in the imaging area of camera 570 directly over thespot. Various other methods will be readily apparent.

Head-Travel Alignment (Step 1120)

A print head having multiple ejectors must be accurately aligned withthe print travel axis (axes) of the IC printing system in which it isused, so that a drop of print solution can be placed at any requiredposition on a substrate within a desired accuracy. If there is angularmisalignment between the print head relative to the print travel axis,the IC pattern produced by the print head will exhibit a correspondingamount of distortion. FIGS. 5 a and 5 b provide detail views of an ICprinting system that depict a method for performing this print head toprint travel alignment according to an embodiment of the invention. InFIG. 5 a, a print assembly 550 is positioned over a substrate 520 on astage 510. Print assembly 550 is substantially similar to print assembly250 shown in FIG. 2, and includes a print head 530 and a camera 570mounted in a rigid mount 560. Print head 530 comprises ejectors540(0)-540(5) arranged in a diagonal line in an ejector base 531. Notethat while six ejectors are depicted, print head 530 can include anynumber and arrangement of ejectors. A single spot 525(0) is printed onsubstrate 520 by a first selected one of ejectors 540(0)-540(5)—in thiscase ejector 540(0). Then, as shown in FIG. 5 b, print assembly 550 istranslated by a distance Dx(1) in the X axis direction and a distanceDy(1) in the Y axis direction, with distances Dx(1) and Dy(1)representing the values expected to position a second selected ejector(in this case ejector 540(5)) over spot 525(0). Ejector 540(5) thenprints a second spot 525(5). Camera 570 can then be used to measure thedistance between spots 525(0) and 525(5), and the orientation of printhead 530 can be adjusted with respect to substrate 520 (or stage 510) tocompensate for the misalignment. Note that this relative rotation ofprint head 530 can be accomplished by actually rotating print head 530within fixed mount 560 (as indicated by the curved arrow), or by leavingthe position of print head 530 unchanged and rotating stage 510. Afterrecalibrating the camera to ejector/drop location, the two ejectors(530(0) and 530(5)) can then be used to print two more drops at a singlelocation to determine if further rotational correction is required. Thistype of iteration can be continued until the spots produced by the twoejectors align within a desired degree of accuracy. This process issummarized in steps 501-506 of the flow chart shown in FIG. 5 c.

FIGS. 6 a and 6 b provide detail views of an IC printing system thatdepict an alternative method for performing a print head to print travelalignment operation, according to another embodiment of the invention.In FIG. 6 a, a print assembly 550 is positioned over a substrate 520 ona stage 510. Print assembly 550 is substantially similar to printassembly 250 shown in FIG. 2, and includes a print head 530 and a camera570 mounted in a rigid mount 560. Print head 530 comprises ejectors540(0)-540(5) arranged in a diagonal line in an ejector base 531. Notethat while six ejectors are depicted, print head 530 can include anynumber and arrangement of ejectors. The position of each ejector550(0)-550(5) relative to an alignment target 571 within the imagingsensor of camera 570 is accurately determined, a relativelystraightforward task since both the camera and ejectors 550(0)-550(5)are held in fixed positions by rigid mount 560. Then, as depicted inFIG. 6 a, a single drop of print solution is deposited from each ofejectors 550(0)-550(5) onto substrate 520 in a single ejection event.This ejection event can be performed at the printing speed to be usedduring actual IC printing so that any effects due to variations inejection velocity can be accounted for. For each ejected spot, camera570 is positioned over the design position (i.e., expected position) ofthe corresponding ejector, as shown in FIG. 6 b, and using machinevision object location methods (such as a center of mass or circularHough transform algorithm), the actual location of the spot is detectedand the X axis and Y axis offsets (Dx(2) and Dy(2), respectively) of thespot from the design position are calculated. The Y axis offset for eachspot can then be plotted against the expected X axis position of thespot (Xexp), as shown by the graph in FIG. 7. A best fit line 700 canthen be calculated, wherein a slope Sa of line 700 indicates the angulardifference between the print head 530 and the X-axis travel of stage510. Note that the X axis offset for each spot plotted against theexpected Y axis position of the spot would provide an alternative meansfor calculation of this angular difference. Print head 530 could then berotated with respect to stage 510 to compensate for this misalignment.The entire procedure could then be repeated until the measured angulardifference falls below a desired threshold angle. This process issummarized in steps 601-607 of the flow chart shown in FIG. 6 c.

Ejector Mapping (Step 1130)

The X axis and Y axis offset data (the collection of which was describedwith respect to FIGS. 6 a and 6 b) can also be used to more accuratelydetermine the spacing between ejectors in the print head duringoperation of an IC printing system. During printing operations, theprint head often must be heated to ensure proper printing solution flow.The resulting thermal expansion of the print head can alter the ejectorspacing from its design (expected) values. According to an embodiment ofthe invention, this thermally-induced offset can be measured andcompensated for in the following manner. First, a set of X axis and Yaxis offsets is determined as described previously with respect to FIGS.6 a and 6 b. Then, X axis offset for each spot can then be plottedagainst the expected X axis position of the spot (Xexp), as shown by thegraph in FIG. 8 a. A best fit line 800 a can then be calculated, whereina slope Sx of line 800 a defines a correction factor to be applied tothe X axis design positions of the ejectors during actual printoperations. The application of this correction factor is as follows:

Xact(n)=Xexp(n)+Sx*(Xexp(n)−Xexp(0))  (1)

where Xact(n) is the actual X axis position of the nth ejector in aprint head, Xexp(n) is the design (expected) X axis position of the nthejector, and Xexp(0) is the design X axis position of a referenceejector in the print head (i.e., the ejector used in the aforementionedcamera-to-print-head position determination). For example, for the graphof FIG. 8 a, Xexp(0) would be the design X axis position of ejector550(0) in FIGS. 6 a and 6 b. In a similar fashion, a slope Sy can bedetermined for a best fit line for a graph of Y axis offset for eachspot plotted against the expected Y axis position of the spot (Yexp), asshown in FIG. 8 b. Slope Sy provides a correction factor for the Y axispositions of the ejectors as follows:

Yact(n)=Yexp(n)+Sy*(Yexp(n)−Yexp(0))  (2)

where Yact(n) is the actual Y axis position of the nth ejector in aprint head, Yexp(n) is the design (expected) Y axis position of the nthejector, and Yexp(0) is the design Y axis position of the referenceejector. In this manner, thermal effects on ejector position can beefficiently corrected. This process is summarized in steps 801-807 ofthe flow chart shown in FIG. 8 c.

Ejector Selection (Step 1140)

The inherent difficulty in producing multi-ejector print heads having alarge number of high-quality ejectors necessitates the use of ejector“selection-filtering” for high accuracy printing (e.g., 40 um dropsplaced with 1 um accuracy and drop uniformity <5%). This problem is onlymagnified when IC printing systems incorporate the readily availableinexpensive print heads intended for document production. Often, only asubset of the ejectors in a print head can perform with the requiredaccuracy (printing tolerance). In such situations, a method for ejectorselection according to an embodiment of the invention involves:performing the camera to print head alignment procedure described above;performing the ejector-to-ejector position testing described above andapplying the appropriate compensation to the layout data; selecting aspot placement accuracy tolerance and a spot size variation tolerance;identifying any other selection parameters unique to the print headdesign (i.e. collinear position redundancy, sparse outlier removal,etc.); and filtering for ejectors that provide the desired printingtolerances within the selection parameters. According to an embodimentof the invention, this filtering can be verified by printing an ejectorpattern using all the filter-selected ejectors. The ejector pattern canbe formed by a single ejection event or by a print operation that printsa predetermined pattern that utilizes all the selected ejectors. Thespot placement for each ejector can then be compared to the selectionparameters to identify those ejectors that provide the desired printingtolerances. Once these “good” ejectors have been selected and verified,the remaining ejectors can be disabled or removed from operation insubsequent printing operations.

Layer Alignment (Steps 1170 and 1180)

Because printing an IC pattern in accordance with the invention willrequire at least two print operations (i.e., one for each of thenon-parallel design layers), some means must be provided for ensuringthat the design layer used in the second print operation is aligned withthe design layer printed in the first print operation. According to anembodiment of the invention, this alignment can be facilitated byprinting a set of alignment marks on the substrate. Then the cameramounted in the print assembly (e.g., camera 270 shown in FIG. 2) can beused to perform a layer alignment so that the layers printed duringdifferent printing operations are properly aligned.

FIG. 9 a depicts a method for performing this layer alignment accordingto an embodiment of the invention. In FIG. 9 a, a substrate 520 isplaced on a rotational platform 512 on a stage 510 and is positionedusing standard positioning features such as a flat 522 on substrate 520and alignment features 511 on stage 510. Alignment marks 521(a)-521(d)have previously been printed in unused locations on substrate 520,either prior to or during printing of the initial design layer.Alignment marks 521(a)-521(d) can be part of the initial design layer,or they can be included in a special alignment design layer. Note thatwhile four alignment marks are depicted, any number of alignment marks(greater than one) could be printed on substrate 520. Note further thatwhile alignment marks 521(a)-521(d) are depicted as cross-shapedelements for explanatory purposes, they can comprise any shape capableof accurately indicating position.

To perform the layer alignment, camera 570 is used to gauge the offset(i.e., a vector distance and direction) between the design position andactual position for each of alignment marks 521(a)-521(d). Thesemeasurements can be taken by moving camera 570 to the design positionand then comparing the actual position of the associated alignment markwith an alignment target 571 in the imaging sensor of camera 570 (theprint assembly that houses camera 570 and its associated print head arenot shown for clarity). After each such measurement, or after aspecified number of such measurements, substrate 520 is rotated byrotational platform 512 (as indicated by the curved arrows). Thismeasurement-rotation sequence is repeated until the offsets for each ofalignment marks 521(a)-521(d) are the same (within a specifiedtolerance).

Then, camera 570 measures the position of each reference mark withrespect to a predefined origin point and averages those measurements toobtain the actual location of an alignment reference point for alignmentmarks 521(a)-521(d) (indicated as location 525(ref)). The alignmentreference point has a known position relative to a design layer originof the first design layer (indicated as location 525(L1)), so once theactual location of the alignment reference point is known, the actuallocation of the design layer origin of the first design layer can bedetermined. According to an embodiment of the invention, the alignmentreference point and the design layer origin of the first design layercan be coincident. The X axis and Y axis offsets, Cx and Cy,respectively, between the actual location of the design layer origin ofthe first design layer and its expected position (indicated as location525(exp)) provide a translation vector that can be applied to the nextdesign layer to be printed so that it is aligned with substrate 520, andhence aligned with the previously printed layers. This process issummarized in steps 901-907 in the flow chart shown in FIG. 9 c.

Note that if the print head only has a single ejector, the rotationalorientation of the print head relative to the substrate is not ascritical, since the single ejector can perform a vector printingoperation. Therefore, while the layer alignment process described abovecould be used for a single-ejector print head, a somewhat simpler layeralignment process could also be used. FIG. 9 b depicts a layer alignmentoperation for a single ejector print head in accordance with anembodiment of the invention. In FIG. 9 b, a substrate 520 is placed on astage 510 and is positioned using standard positioning features such asa flat 522 on substrate 520 and alignment features 511 on stage 510.Alignment marks 521(a)-521(d) have previously been printed in unusedlocations on substrate 520, either prior to or during printing of theinitial design layer. Alignment marks 521(a)-521(d) can be part of theinitial design layer, or they can be included in a special alignmentdesign layer. Note that while four alignment marks are depicted, anynumber of alignment marks (greater than one) could be printed onsubstrate 520. Note further that while alignment marks 521(a)-521(d) aredepicted as cross-shaped elements for explanatory purposes, they cancomprise any shape capable of accurately indicating position. Camera 570(the print assembly that houses camera 570 and its associated print headare not shown for clarity) measures the actual locations of alignmentmarks 521(a)-521(d) and determines their offsets from the designalignment mark positions. For example, the offsets in the X axis and Yaxis directions for reference mark 521(a) are offsets R×(a) and Ry(a),respectively. The measured offsets for all of reference marks521(a)-521(d) can then be used to calculate an angle of rotation andtranslation vector to be applied to the next design layer to be printedon substrate 520. This process is summarized in steps 991-995 of theflow chart shown in FIG. 9 d.

Printing with Sparse Ejectors

Printing with a print head in which only a portion of the ejectors isactive (i.e., a “sparse ejectors” condition) requires that the controllogic account for the missing (disabled) ejectors during the printingoperations. FIGS. 10 a and 10 b depict a method for printing with asparse set of ejectors in accordance with an embodiment of theinvention. FIG. 10 a shows a rasterized image 1000 of a design layer tobe printed by a print head 1030. Note that while print head 1030includes six ejectors 1050(0)-1050(5) in a diagonal line for explanatorypurposes, print head 1030 can include any number of ejectors in anyarrangement. Rasterized image 1000 includes raster columns a-t and rowsR01-R13. The horizontal spacing Xoff and the vertical spacing Yoffbetween each of ejectors 1050(0)-1050(5) are selected to be an integermultiples of the raster column and row spacings of rasterized image1000. For example, spacing Xoff is four times the raster column spacing,while spacing Yoff is two times the raster row spacing. The data to beprinted in raster columns a-t are indicated by shaded regions—e.g.,raster data blocks 1001 and 1002 in raster column a. Note that while theprint direction is defined to be in the vertical direction forexplanatory purposes, the same principles would apply to a horizontalprint direction.

The sparse set of ejectors for print head 1030 is defined as all theejectors except ejector 1050(3), which is marked with an “X” to indicatethat it does not meet the specified ejector performance requirements.The first raster column containing valid data is then selected (column ain this example), and that column is set to correspond to the firstejector (ejector 1050(0) in this example). The positions of theremaining “good” ejectors 1050(1), 1050(2), 1050(4), and 1050(5) thendetermine the additional raster columns to be printed; i.e., columns d,g, m, and p, respectively. The print data is extracted from columns a,d, g, m, and p, and associated with ejectors 1050(0), 1050(1), 1050(2),1050(4), and 1050(5), respectively, for the next printing pass. Notethat since ejectors 1050(0)-1050(5) are offset from one another in thevertical direction, a corresponding vertical offset must be applied tothe extracted raster data to ensure that the printed features areproperly positioned. For example, since ejector 1050(1) is two rasterrows above ejector 1050(0) in the vertical direction (offset Yoff istwo), the print data associated with ejector 1050(1) must be shiftedahead by two to ensure that the printed features are properly aligned.

FIG. 10 b shows the results of the first printing pass performed in FIG.10 a. Raster columns a, d, g, m, and p have been emptied of data by theextraction operations performed with respect to FIG. 10 a, so that thefirst column in updated rasterized image 1000′ having valid data is nowcolumn b (which includes raster data blocks 1003-1008). Therefore,column b is set to correspond to ejector 1050(0), which in turn definesthe correlation between ejectors 1050(1), 1050(2), 1050(4), and 1050(5)and raster columns e, h, n, and q, respectively. The print data isextracted from these new columns, the data is adjusted by theappropriate vertical offset to compensate for the vertical offsetbetween ejectors, and a second printing pass is performed. Printingusing the sparse ejectors of print head 1030 continues in this manneruntil no valid data remains in the rasterized image. This process issummarized in steps 1001-1007 of the flow chart shown in FIG. 10 c.

Although the present invention has been described in connection withseveral embodiments, it is understood that this invention is not limitedto the embodiments disclosed, but is capable of various modificationsthat would be apparent to one of ordinary skill in the art. Therefore,the invention is limited only by the following claims.

1. A method for forming a printed pattern from a pattern layout using aprinting system, the printing system comprising a print head and acamera rigidly connected to the print head, the camera having an imagingsensor, the method comprising: performing a first alignment operation toachieve a specified orientation between the print head and a first setof alignment marks on a substrate using first image data generated bythe imaging sensor of the camera; performing a first print operationafter the first alignment operation to print a first portion of thepattern layout on the substrate to generate a first portion of theprinted pattern, wherein performing the first print operation includesmaking a plurality of printing passes across the substrate in a firstprint direction; performing a second alignment operation to orient theprint head relative to a second set of alignment marks on the substrateusing second image data generated by the imaging sensor of the camera;and performing a second print operation to print a second portion of thepattern layout on the substrate to generate a second portion of theprinted pattern, wherein performing the second print operation includesmaking a plurality of printing passes across the substrate in a secondprint direction that is non-parallel to the first print direction. 2.The method of claim 1, wherein the first portion of the pattern layoutcomprises a first plurality of layout elements from the pattern layout,wherein substantially all of the first plurality of layout elements runparallel to a first reference axis, wherein performing the first printoperation comprises making the plurality of printing passes across thesubstrate in the first print direction such that the first printdirection is aligned with the first reference axis, wherein the secondset of alignment marks are the same as the first set of alignment marks.3. The method of claim 1, wherein the second portion of the patternlayout comprises a second plurality of layout elements from the patternlayout, wherein substantially all of the second plurality of layoutelements run parallel to a second reference axis, and wherein performingthe second print operation comprises making the plurality of printingpasses across the substrate in the second print direction such that thefirst print direction is aligned with the second reference axis.
 4. Themethod of claim 1, wherein the print head comprises a first ejector anda second ejector, and wherein performing the first alignment operationcomprises: positioning the first ejector at a first location over thesubstrate and printing a first spot on the substrate; positioning thesecond ejector at the first location and printing a second spot on thesubstrate; measuring a distance between the first spot and the secondspot; adjusting a rotational orientation between the print head and thesubstrate to reduce the distance between the first spot and the secondspot; and repeating the steps of positioning the first ejector,positioning the second ejector, measuring the distance, and adjustingthe rotational orientation until the distance between the first spot andthe second spot is less than a predefined threshold value.
 5. The methodof claim 1, wherein the print head includes a plurality of ejectors,wherein the imaging sensor of the camera comprises an alignment target,and wherein performing the first alignment operation comprises:determining a relative position between the alignment target and each ofthe plurality of ejectors; positioning the print head over thesubstrate; printing a spot onto the substrate from each of the pluralityof ejectors, each of the spots printed by the plurality of ejectorshaving an actual location with reference to a horizontal axis and avertical axis parallel to the substrate; determining a vertical offsetbetween an expected location of each spot along the vertical axis andthe actual location of the spot along the vertical axis; calculating alinear fit line for the vertical offset of each spot plotted against anexpected location of the spot along the horizontal axis; calculating theslope of the linear fit line; and rotating the print head relative tothe substrate according to an angle defined by the slope of the linearfit line.
 6. The method of claim 5, wherein determining the verticaloffset between the expected location of each spot along the verticalaxis and the actual location of the spot along the vertical axiscomprises: positioning the substrate relative to the camera such thatthe alignment target is aligned with the expected location of each spot;and detecting the actual location of the spot using a machine visionobject location method to determine a difference between the actuallocation and the expected location.
 7. The method of claim 1, whereineach of the set of alignment marks has an expected location, and whereinperforming the first alignment operation comprises: measuring an actuallocation for each of the plurality of alignment marks; comparing theactual location for each of the plurality of alignment marks with theexpected location for each of the plurality of alignment marks todetermine an angle of rotation and translation vector; and utilizing theangle of rotation and translation vector to align the print head withthe substrate.
 8. The method of claim 1, further comprising printing thefirst set of alignment marks using the print head prior to the firstalignment operation.
 9. The method of claim 1, wherein the printedpattern comprises one of an integrated circuit and a semiconductorprocess mask.
 10. A printing system for printing a pattern layout onto asubstrate, the printing system comprising: a stage for supporting thesubstrate; a print head; a camera rigidly connected to the print head,the camera having an imaging sensor; a positioning mechanism fortranslating the print head relative to the substrate; an alignmentmechanism for adjusting a rotational orientation between the print headand substrate; and a system controller for controlling the positioningmechanism and the alignment mechanism, wherein the system controllercomprises: logic for performing a first alignment operation to achieve aspecified orientation between the print head and a first set ofalignment marks on the substrate in response to first image datagenerated by the imaging sensor of the camera; logic for performing afirst print operation after the alignment operation to print a firstportion of the pattern layout on the substrate such that the first printoperation includes making a plurality of printing passes across thesubstrate in a first print direction; logic for performing a secondalignment operation to achieve a specified orientation between the printhead and a second set of alignment marks on the substrate in response tosecond image data generated by the imaging sensor of the camera; andlogic for performing a second print operation to print a second portionof the pattern layout on the substrate such that the second printoperation includes making a plurality of printing passes across thesubstrate in a second print direction that is non-parallel to the firstprint direction.
 11. The printing system of claim 10, wherein the firstportion of the pattern layout comprises a first plurality of layoutelements from the pattern layout, wherein substantially all of the firstplurality of layout elements run parallel to a first reference axis,wherein the logic for performing the first print operation compriseslogic for causing the positioning mechanism to make the plurality ofprinting passes across the substrate in the first print direction suchthat the first print direction is aligned with the first reference axis,and wherein the second set of alignment marks are the same as the firstset of alignment marks.
 12. The printing system of claim 10, wherein theprint head comprises a first ejector and a second ejector, and whereinthe logic for performing the alignment operation comprises: logic forpositioning the first ejector at a first location over the substrate andprinting a first spot on the substrate; logic for positioning the secondejector at the first location and printing a second spot on thesubstrate; logic for measuring a distance between the first spot and thesecond spot; logic for causing the alignment mechanism to adjust therotational orientation between the print head and the substrate toreduce the distance between the first spot and the second spot; andlogic for repeatedly executing the logic for positioning the firstejector, the logic for positioning the second ejector, the logic formeasuring the distance, and the logic for causing the alignmentmechanism to adjust the rotational orientation between the print headand the substrate until the distance between the first spot and thesecond spot is less than a predefined threshold value.
 13. The printingsystem of claim 10, wherein the print head includes a plurality ofejectors, wherein the imaging sensor of the camera comprises analignment target, and wherein the logic for performing the alignmentoperation comprises: logic for determining a relative position betweenthe alignment target and each of the plurality of ejectors; logic forcausing the print head to be positioned over the substrate such thateach of the plurality of ejectors is at an expected location withreference to a horizontal axis and a vertical axis parallel to thesubstrate; logic for causing the print head to printing a spot onto thesubstrate from each of the plurality of ejectors, each of the spotsprinted by the plurality of ejectors having an actual location withreference to the horizontal axis and the vertical axis; logic fordetermining a vertical offset between an expected location of each spotalong the vertical axis and the actual location of the spot along thevertical axis; logic for calculating a linear fit line for the verticaloffset of each spot plotted against an expected location of the spotalong the horizontal axis; logic for calculating the slope of the linearfit line; and logic for causing the alignment mechanism to adjust therotational orientation of the print head relative to the substrateaccording to an angle defined by the slope of the linear fit line. 14.The printing system of claim 10, wherein the logic for performing thealignment operation comprises: logic for measuring an actual locationfor each of a plurality of alignment marks on the substrate; logic forcomparing the actual location for each of the plurality of alignmentmarks with an expected location for each of the plurality of alignmentmarks to determine an angle of rotation and translation vector; andlogic for utilizing the angle of rotation and translation vector tocause the positioning mechanism and the alignment mechanism to align theprint head with the substrate.